The invention relates to locating an elongated object in a three-dimensional array of data, and especially, but not exclusively, to locating an elongated structure in a tomographic imaging dataset. The invention has particular application to locating the nerve canal in x-ray tomographic dental imaging of the mandible or lower jaw of a human or other mammal.
In certain forms of dental surgery, such as implant surgery, it is necessary to drill into the mandible. For secure implantation of, for example, a prosthetic tooth, a deep drilled hole is desirable. However, within the mandible there is a nerve canal containing the neurovascular bundle to the teeth and parts of the face. If the surgeon drills too deeply, and damages the nerve, permanent numbness of portions of the face may result, to the potentially considerable detriment of the patient.
A set of three-dimensional data relating to a property of an object that varies over space within the object may be obtained in various ways. For example, an x-ray image of a target may be obtained by placing the target between a source of x-rays and a detector of the x-rays. In a computed tomography (CT) system, a series of x-ray images of a target are taken with the direction from the source to the detector differently oriented relative to the target. From these images, a three-dimensional representation of the density of x-ray absorbing material in the target may be reconstructed. Other methods of generating a three-dimensional dataset are known, including magnetic resonance imaging, or may be developed hereafter.
From the three-dimensional data, a section in a desired plane may be generated. In planning for dental implant work, the surgeon may display tomograms in planes along and across the mandible, and attempt to identify the nerve canal in the tomograms. However, the contrast of x-ray tomograms is often poor, and the interior of the mandible is formed from bone of comparatively low density. In addition, the nerve canal is not smoothly curved along its length, but undulates irregularly. The nerve may therefore appear in an unexpected place in a transverse section, and may be difficult to follow in a longitudinal section. The nerve may also disappear out of the plane of a longitudinal section regardless of how the section plane is aligned. Consequently, it is not always easy for the surgeon to recognize the mandibular nerve and plan surgery correctly.
There is therefore a hitherto unfulfilled need for a system by which the mandibular nerve can be accurately identified and marked on tomograms of the mandible, enabling the surgeon reliably to avoid the nerve when drilling for implantation, and to assess whether sufficient depth of sound bone is available for a good implant.